Oximeters provide useful measurements of oxyhemoglobin saturation for clinical purposes and for physiological studies of oxygen transport. Unfortunately, presently available oximeters suffer from several serious disadvantages. They are often bulky, expensive instruments designed primarily for hospital use, and they can be particularly inaccurate in the low oxygen saturation range. In addition, known oximeters typically destroy the blood sample by diluting or hemolyzing it before measurement thereby rendering the blood sample useless for further study. Those oximeters that use whole, undiluted blood must be recalibrated if the total hemoglobin concentration changes.